Anchoring covalent organic framework onto biochar provides novel amendment for immobilizing cadmium and lead pollutants in soil

The escalating crisis of heavy metal contamination in soil demands advanced material to effectively control pollution risk. To enhance the effectiveness of biochar to remediate the soils contaminated with cadmium (Cd) and lead (Pb), this study developed an in-situ polymerization method to anchor covalent organic frameworks (COF), one class of emerging porous polymers, onto rice husk biochar surfaces, generating a novel COF-modified rice husk biochar (COF-RB). Adsorption studies showed that COF-RB exhibited a significantly faster adsorption rate and higher adsorption capacity for Cd²⁺ and Pb²⁺ compared to RB. The maximum adsorption capacities of COF-RB for Cd²⁺ and Pb²⁺ were 40.56 and 101.33 mg g⁻¹, respectively, approximately 7-fold and 5-fold higher than RB alone. The metal adsorption on COF-RB fit the Freundlich isotherm model and the pseudo-second-order kinetic model, indicating the occurrence of chemical adsorption. X-ray photoelectron spectroscopy (XPS) analysis further revealed that the surface complexation with nitrogen-containing functional groups on COF-RB played an essential role in facilitating the Cd²⁺ and Pb²⁺ adsorption process. When COF-RB was applied into contaminated soil, both the concentrations of soil available Cd and Pb decreased, ranging from 42.70 %–65.42 % and 23.29 %–45.78 %, respectively, compared to RB-treated soil. Additionally, COF-RB promoted the transformation of the metals from labile to more stable fractions in soils. Overall, the findings highlight that COF-RB presents as a promising solution for immobilizing Cd and Pb pollutants in water and soils. The research offers new insight and method for biochar surface modification, and facilitates the remediation of heavy metal contamination.